There is an interesting article on placebos in last week's New Scientist.
Placebos are a vital element of Randomized Clinical Trials (RCT) because the very act of seeing a doctor and being prescribed a treatment can make a patients feel better. To avoid this placebo effect, an inert substance is given to the control group. The more spectacular the placebo procedure the more beneficial the effect. The most dramatic placebo effect I have heard about is sham plasmapheresis, where a machine with many flashing lights attended by a pretty nurse, removes the patient's plasma by a sort of spin-drying effect and replaces it with donor plasma. Twenty years ago a doctor using this technique got himself on the front page of Time Magazine because of his remarkable success in alleviating rheumatoid arthritis.
Unfortunately, it was later shown that the same effect could be obtained by sham plasmapheresis, where the patient's own plasma was returned after the same dramatic procedure. Recently sham acupuncture - where no concern was made to place the needles in the Chinese meridians - was found to be equally as effective as proper acupuncture.
So not all placebos are equal was one of the messages of the article. The other was the difficulty in keeping a trial double blinded. If patients are given an inert substance as the placebo, they will soon recognize it because they will have no side effects. One solution to this is to use a placebo that gives the side effects but not the benefit. Atropine has been suggested as a placebo in trials of antidepressants since it produces the same side effects - dry mouth, headaches, insomnia and drowsiness. But would it be ethical? I don't think so.
Perhaps it would be better simply to ask the patients whether they thought they were receiving active drug or the placebo. At least then we would know the extent of the unblinding that had taken place.